March 24, 2016

Analysts at the Energy Department's National Renewable Energy Laboratory (NREL) have
used detailed light detection and ranging (LiDAR) data for 128 cities nationwide,
along with improved data analysis methods and simulation tools, to update its estimate
of total U.S. technical potential for rooftop photovoltaic (PV) systems. The analysis
reveals a technical potential of 1,118 gigawatts (GW) of capacity and 1,432 terawatt-hours
(TWh) of annual energy generation,
equivalent to 39 percent of the nation's electricity sales.

This current estimate is significantly greater than that of a previous NREL analysis,
which estimated 664 GW of installed capacity and 800 TWh of annual energy generation.
Analysts attribute the new findings to increases in module power density, improved
estimation of building suitability, higher estimates of the total number of buildings,
and improvements in PV performance simulation tools.

To calculate these estimates, NREL analysts used LiDAR data, Geographic Information
System methods, and PV-generation modeling to calculate the suitability of rooftops
for hosting PV in 128 cities nationwide-representing approximately 23 percent of U.S.
buildings-and provide PV-generation results for 47 of the cities. The analysts then
extrapolated these findings to the
entire continental United States. The result is more accurate estimates of technical
potential at the national, state, and zip code level.

"This report is the culmination of a three-year research effort and represents a significant
advancement in our understanding of the potential for rooftop PV to contribute to
meeting U.S. electricity demand," said Robert Margolis, NREL senior energy analyst
and co-author of the report.

Within the 128 cities studied, the researchers found that 83 percent of small buildings
have a suitable location for PV installation, but only 26 percent of those buildings'
total rooftop area is suitable for development. Because of the sheer number of this
class of building across the country, however, small buildings actually provide the
greatest combined technical potential. Altogether, small building rooftops could accommodate
up to 731 GW of PV capacity and generate 926 TWh per year of PV energy-approximately
65 percent of the country's total rooftop technical potential. Medium and large buildings
have a total installed capacity potential of 386 GW and energy generation potential
of 506 TWh per year, approximately 35 percent of the total technical
potential of rooftop PV.

"An accurate estimate of PV's technical potential is a critical input in the development
of regional deployment plans," said Pieter Gagnon, an engineering analyst of solar
policy and technoeconomics at NREL and lead author of the report. "Armed with this
new data, municipalities, utilities, solar energy researchers, and other stakeholders
will have a much-improved starting point
for PV research and policymaking, both regionally and nationwide."

"It is important to note that this report only estimates the potential from existing,
suitable rooftops, and does not consider the immense potential of ground-mounted PV,"
said Margolis. "Actual generation from PV in urban areas could exceed these estimates
by installing systems on less suitable roof space, by mounting PV on canopies over
open spaces such as parking lots, or by
integrating PV into building facades. Further, the results are sensitive to assumptions
about module performance, which are expected to continue improving over time."

Technical potential is an established reference point for renewable technologies.
It quantifies the amount of energy that can be captured from a particular resource,
considering resource availability and quality, technical system performance, and the
physical availability of suitable area for development-without consideration of economic
factors like return on investment or market factors such as policies, competition
with other technologies, and rate of adoption.

NREL's work was supported by funding from the Energy Department's Office of Energy
Efficiency and Renewable Energy in support of its SunShot Initiative. The SunShot
Initiative is a collaborative national effort that aggressively drives innovation
to make solar energy fully cost-competitive with traditional energy sources before
the end of the decade. Through SunShot, the department supports efforts by private
companies, universities, and national laboratories to drive down the cost of solar
electricity to $0.06 per kilowatt-hour. Learn more at energy.gov/sunshot.

NREL is the U.S. Department of Energy's primary national laboratory for renewable
energy and energy efficiency research and development. NREL is operated for the Energy
Department by The Alliance for Sustainable Energy, LLC.